This invention relates to optical systems, and, more particularly, to a blurring array that defocuses the optical beam in a controlled manner and avoids interference effects in the defocused beam.
In one common configuration, an imaging infrared (IR) optical sensor (i.e., a focal plane array or FPA detector) consists of a large number, typically thousands or tens of thousands, of individual electro-optic detector elements, which are positioned at the focal plane of the optical system. The detector elements view a scene through an appropriate optical path and produce an electrical output responsive to the scene. The materials and/or construction of the detector elements may be selected to be sensitive to different wavelength ranges (wavebands) of electromagnetic radiation, including, for example, infrared energy. The detector elements are arranged in a planar array, with each detector element providing one pixel of an image. The outputs of the detector elements are digitally processed to form an electronic re-creation of the image which may be further analyzed.
Ideally, all of the detector elements would respond identically to incident energy, with the signal output of each detector element identically proportional to the incident energy. In practice within the limitations of current technology, however, each of the different detector elements may be expected to respond slightly differently. Changes in the pixel responses may also develop in the detector array with time, cooling variation, or use. These differences may be evident as gain or zero-point offsets, nonlinearity, or other types of departures from the ideal identical response. As a result of such departures, if a perfectly uniform infrared input scene were presented to the FPA detector, the detector output would not be perfectly uniform.
A variety of techniques are known for both reducing the departure from the ideal in the mass-produced detector elements during production, and also for compensating for non-ideal responses which develop during service. Calibration and real-time service techniques have been developed to compensate for the nonuniformities in the detector elements. Some of these techniques require a controllable blurring of the image, such that during some periods an unblurred image is viewed by the detector and during other periods a blurred image is viewed by the detector. Effective system performance requires the blurred image to be free of significant structure due to scene features that are insufficiently smoothed or artifacts created by interference effects. A substantial reduction in mean incident energy reaching the detector also has an adverse effect, so the blurring must not be so great as to divert significant energy outside the sensitive region of the detector, and large-angle scatter must be controlled.
The controlled blurring of the scene presents a challenge, and a variety of techniques have been used. For example, with one known approach, the scene is viewed by the detector array through two optically transparent elements of different optical path lengths (thickness and/or refractive index). One element is selected such that the image is focused onto the focal plane of the detector array, and the other element is selected such that the focus is longitudinally displaced from the focal plane of the detector array. While operable, in practice it has been found for infrared sensors that the blurring is insufficient, and that relatively high spatial frequency features of the scene may still be discerned.
Another approach, described in U.S. Pat. No. 5,867,307, improves upon the prior techniques and provides adequate blurring performance in many applications. However, there are some situations where interference effects are present in the blurred image, or there may be too much blurring, resulting in an unacceptable loss of scene energy.
Accordingly, there is a need for an improved approach to intentional and controlled image blurring for infrared detectors, particularly in systems with restrictive space constraints such as infrared missile seekers. The present invention fulfills this need, and further provides related advantages.
The present approach provides an optical system with a blurring array. The controlled blurring approach is applicable to optical systems of a wide range of wavelengths but is of particular interest for infrared systems. The blurring array provides a controllable degree of blurring. Additionally, it avoids interference effects in the blurred image that may lead to false images being detected by the detector. The result is that the input image of the scene may be controllably blurred for both calibration and service requirements. The blurring array is designed for compatibility with available manufacturing techniques.
In accordance with the invention, an optical system comprises a blurring array comprising a substrate having a surface array of blurlets whose optical foci vary from a nominal focal surface location and/or whose optical phase at the nominal focal surface location varies in a pseudorandom but deterministic manner. The blurlet is an optical element that introduces a controlled blurring into an image. The blurlet may be a lenslet in a substrate material transmissive to a waveband of incident infrared energy, such as silicon for the mid-wavelength infrared range of about 3-5 micrometers wavelength. The blurlet may instead be a mirrorlet that reflects optical energy. In one application, the infrared system further includes a detector lying at about the nominal focal surface location, and an optics system that focuses infrared energy from a scene onto the detector.
An array of identical or nearly identical blurlets would focus the incident beam to a blurlet focal plane slightly different from that of the detector. There would be blurring, but the inventors have found that interference effects typically result from such a regular array of blurlets. The present approach varies the foci and/or phase of the energy passing through the individual blurlets in a pseudorandom fashion. That is, there is a random variation within constraints imposed in the design process.
In a typical case, the foci of the blurlets and/or their axial offsets are at locations relative to the nominal focal surface location that are randomly selected from a set of values defined by a distribution, more preferably a truncated distribution that prevents too wide a spread in the values. For example, the distribution may be truncated at +/xe2x88x92 one standard deviation (or other fixed value) on either side of the mean, which prevents too great a blurring of the image. A variety of distributions, such as normal, flat, and other distributions may be used. A normal distribution was convenient for use in the preferred embodiments, but the practice of the invention is not so limited. The blurlets may be surfaces each defined by a curvature and an axial offset from the nominal focal surface location, wherein at least one of the curvature and the axial offset of each blurlet is a value that is randomly selected from a set of values defined by a respective distribution or respective truncated distribution.
The present invention thus provides a blurring array that achieves a controllable amount of blurring and also avoids interference effects in the blurred image, without excessively reducing the scene energy that reaches the detector. Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.